A frequency modulated continuous wave (FMCW) radar can be designed to transmit and receive from the same antenna, or to transmit and receive from separate antennas. Using separate antennas for transmitting and receiving provides improved isolation between the transmit and receive path at the expense of increased size to accommodate each of the antennas and provide spacing between the antennas. In order to reduce the size, radars can be designed to transmit and receive from a single antenna. Using a single antenna, however, decreases the isolation between the transmit and receive path resulting in increased coupling of signals from the transmit path into the receive path. These signals from the transmit path appear as noise in the receive path and reduce the sensitivity of the receiver. If enough signal from the transmit path is coupled into the receive path, the transmit signal can drown out the receive signal entirely. In high precision radar applications, such as in radar altimeters for aircraft, radar sensitivity can be crucial.
The frequency range in which a radar operates also has an effect on the performance of the radar. The frequency range used by a radar can be based on available spectrum as well as the distance that the radar will be measuring. For example, aircraft radar altimeters can be required to measure a wide range of distance, from 15,000 ft down to 5 ft. Also, commercial radar altimeters are currently required to operate between 4.2-4.4 GHz.